The present invention relates to a high frequency circuit device using a high frequency circuit element such as a surface acoustic wave (SAW) element, and more particularly to a high frequency circuit device for use in mobile communications which is required to be compact and have high performance filter characteristics.
Conventionally, a high frequency circuit device has generally been produced by mounting a high frequency circuit element in an air-tight sealable TO-can package and providing a matching circuit outside the high frequency circuit element by the use of a printed circuit element or a lumped circuit element, as disclosed, for example, in JP-A-3-205908 (laid open on Sep. 9, 1991). A surface acoustic wave element will be explained below as an example of the high frequency circuit element.
FIG. 2A is a cross-sectional view showing how a conventional surface acoustic wave device is mounted, and FIG. 2B shows a bottom view of the same. A package 7 is mounted on a high frequency circuit substrate (mother board) 16, and a matching circuit is realized by forming an inductive printed circuit element 15, for example, on the rear surface of the high frequency circuit substrate 16.
While FIGS. 2A, 2B illustrate a TO-can package, the structure is roughly identical to the surface mounted device shown in FIG. 3. A surface mounting package 7 shown in FIG. 3 has a substrate 1, a frame 2 formed to stand on a surface of the substrate 1, and a lid 3 for providing, in combination with the frame 2 and substrate 1, an air-tight space in which a high frequency circuit element 8 is disposed. Signal electrodes on the element are connected with a signal electrode pattern 6 on the substrate 1 via wires 9, while grounding electrodes on the element are connected with a grounding electrode pattern on the substrate 1. Such a surface mounting package also needs an external matching circuit, as in the case of the TO-can package.
Generally, the impedance, viewed from input/output terminals of a surface acoustic wave element, is capacitive due to a capacitance between electrodes of a transducer for exciting the surface acoustic wave. For this reason, if a surface acoustic wave element is mounted on a mother board of a terminal of communication equipment or the like, high frequency signals are reflected at an end of the surface acoustic wave element due to a mismatch of impedance between the high frequency signals supplied from the mother board and the surface acoustic wave element. To avoid such an impedance mismatch, a matching circuit is provided on the mother board to cancel capacitive components of the surface acoustic wave element.
FIGS. 4A, 4B, 4C and 4D show examples of surface acoustic wave elements 20 and representative matching circuits, in each of which the surface acoustic wave element is connected with external circuits through matching circuits, each composed of inductive circuit elements or inductive and capacitive circuit elements. As the inductive circuit element, a meander line on the mother board is used, while a lumped capacitance or gap capacitance is used as the capacitive circuit element.
U.S. application Ser. No. 07,924,091 filed on Aug. 3, 1992 discloses a high frequency circuit device in which a film-shaped inductance is disposed in an air-tight space where a high frequency circuit element is disposed.